Spark tube having activated thermionic electrodes



p 27, 1966 H. L. WATTENBACH 3,275,875

SPARK TUBE HAVING ACTIVATED THERMIONIC ELECTRODES Filed Dec. 27. 1962 AAKAL/A/zf EARTH OXIDE AND Mfr/4L PRE/[RABLY lA/CAUDl/VG BAR/UM 0X/0E AND 549/044 Invervtov. Hans L. wa t tenbach I-Iis A' t trrmeg United States Patent 3,275,875 SPARK TUBE HAVING ACTIVATED THERMIONIC ELECTRODES Hans L. Wattenbach, Cleveland Heights, Ohio, assignor t; (lieneral Electric Company, a corporation of New Filed Dec. 27, 1962, Ser. No. 247,633 5 Claims. (Cl. 313185) This invention relates to a spark tube particularly useful in a starting circuit for are lamps requiring a starting voltage much higher than the operating voltage.

In any copending application Serial No. 247,631, filed of even date herewith, now US. Patent No. 3,235,769, entitled, Starting Circuit for Discharge Lamps, and assigned to the same assignee as the present invention, there is described a pulse starting circuit for a metal vapor alumina lamp. This lamp utilizes an envelope of polycrystalline alumina ceramic. The high melting point of this ceramic (over 1900 C.) makes feasible a high pressure sodium vapor lamp having a relatively small diameter and operating at high current density. However, the small diameter increases the breakdown voltage of the lamp and makes starting relatively difiicult. By Way of example, a sodium vapor alumina lamp having an interelectrode distance of 50 to 60 millimeters and an inside diameter of 6 millimeters, has an arc voltage drop from 120 down to 50 volts at operating currents in the range from 6 up to 12 amperes but requires a starting voltage of approximately 1100 volts.

In the starting circuit of my above application, there is included a pulse switching element which operates with a pulse transformer having primary and secondary windings and a pair of capacitances. The secondary is connected, in series with the lamp, across the output terminals of a conventional ballast which functions to limit the lamp current in operation. One capacitance is connected across the ballast output terminals to form a charging circuit. The other capacitance'is connected in series with the primary and the pulse switching element across the ballast output terminals to form a discharging circuit. The pulse switching element fires at every half cycle of the alternating current supply and suddenly discharges the one capacitor into the other through the primary of the pulse transformer. A high voltage low energy pulse is thereby generated in the secondary of the pulse transformer which is applied in series with the line voltage across the lamp electrodes. As soon as the lamp has started, the pulse switching element ceases to fire and the circuit becomes quiescent. An object of my invention is to provide a new and improved spark tube pa.rticularly suitable for the pulse switching element in the above circuit.

To operate successfully in the circuit, the spark tube must meet certain conditions. Its breakdown voltage must be within the peak value of the open circuit voltage provided across the charging circuit. However after the lamp has started and the output of the ballast has decreased from its open circuit voltage to the operating voltage across the lamp, the spark tube must no longer fire. This requires that its breakdown voltage be greater than the operating voltage across the lamp. In order to obtain the breakdown of the tube and the occurrence of the spark at the same phase angle at every half cycle of the line voltage, the breakdown voltage must stay constant. This entails that the initial conditions existing inside the spark tube before breakdown, particularly fill gas pressure and electrode temperature, must be restored after every breakdown within the remaining portion of the half cycle. For optimum circuit eificiency, the spark tube should transmit high current pulses with minimum voltage drop so that the internal dissipation of energy ice can pass large currents at a relatively low arc voltage drop or maintaining voltage at each firing.

In accordance with my invention, I provide a spark tube utilizing activated thermionic type electrodes of refractory metal forming a compact structure having high heat conductivity and defining a relatively short are gap within an envelope containing an inert gas at a moderately low pressure. Suitably the gas filling may consist of argon at a pressure in the range of to 500 millimeters of mercury. In a preferred embodiment, the electrodes consist of tungsten shanks with slipover coils disposed side-by-side and with a spacing between them not exceeding a few millimeters. The electrodes are activated by alkaline earth oxides or interoxides including barium which is applied as a coating and also as a filling in the interstices between turns of the coiling.

The emission mix on the surface of the electrodes causes the arc to create a microscopic hot spot within a very short time, for instance about 10' second, which makes possible a high current density low voltage discharge or so-called spark. After each spark occurrence, this hot spot disappears rapidly by reason of the good thermal conductivity of the electrode structure to which the emission mix is attached so that initial electrode conditions are restored before the end of the half cycle. The narrow gay between the electrodes eliminates most of the positive column of the discharge are so that the voltage drop across the discharge is practically the sum of the cathode and anode voltage falls. As a result of the foregoing, only a small part of the energy stored in the capacitors of the firing circuit is converted into losses inside the spark tube and only a small portion of the volume of the fill gas is ionized and heated up by the discharge. such low losses are easily dissipated by the electrodes and by the fill gas within the half cycle, so that they raise the average temperature of the spark tube by only a few degrees and the deionization of the fill gas occurs within a few microseconds after the discharge. In this way, the initial conditions inside the spark tube are restored before the end of each half cycle with the result that the breakdown voltage remains constant. I have found that such a spark tube may achieve a ratio of breakdown to maintaining voltage of 10 to 1 or better, resulting in extremely efiicient pulse transmission with minimum internal dissipation of energy.

For further objects and advantages and for .a better understanding of the invention, attention is now directed to the followingdes-cription of a preferred embodiment. The features believed to be novel will be more particularly pointed out in the appended claims.

In the drawing wherein like symbols denote corresponding elements throughout the several figures:

FIG. 1 is a schematic diagram of a starting circuit wherein the spark tube of the invention is used.

FIG. 2 is a side elevation of .a spark tube embodying the invention.

FIG. 3 is an enlarged cross sectional View through one of the electrodes of the spark tube.

Referring to FIG. 1, the illustrated starting and operating circuit has line or input terminals 1, 2 for energizetion from the usual 120 volt, 60 cycle alternating 7 current supply. The ballast consists merely of an iron core reactor 3 which is serially inserted between terminal 1 and ballast output terminal 4, the other ballast output terminal 5 being directly connected to terminal 2. Where 3. step-up of the line voltage is required in order to operate the lamp, the ballast will of course include transforming means in addition to series reactance for regulating the discharge current. The pulsing circuit comprises pulse transformer 6 having a primary winding 7 and a secondary winding 8 which is connected between terminal 4 and lamp terminal 9. The other lamp terminal 10 is conductively joined to terminals and 2; the lamp 11, suitably a high pressure sodium vapor alumina lamp,is connected across terminals 9, 10. The pulsing circuit includes a pair of capacitors C C preferably of equal value or rating. Capacitor C is connected across ballast output terminals 4, 5 to form a charging circuit. Capacitor C is connected, in series with a pulse switching ele ment consisting of a spark tube 12 and primary winding 7 of the pulse transformer, across terminals 4, 5 to form a discharging circuit.

The voltage across capacitor C follows substantially the line voltage whereas th-at across capacitor C cannot change so long as spark tube 12 is not conducting. At the instant in the half cycle when the breakdown volt-age of the spark tube is attained, the spark tube conducts current with a maintaining voltage which may be as low as 10 to volts. The spark tube is thus able to transfer high current pulses while only a small part of the pulse energy is dissipated internally. The spark tube.

may be compared to a 'very fast switch wherein the time required for turn-on is about 10 second. At the end of each oscillatory discharge, the spark tube extinguishes until breakdown is again attained during the next half cycle.

The construction of the spark tube 12 is illustrated in FIG. 2.. It comprises a tubular glass bulb 13 having its upper end 14 closed and its lower end peripherally sealed to a glass stem tube 15. Lead-in wires 16,17. extend through the press 18 of the, stem and have the ends of their inward projections bent over to facilitate the weldingthereto of the electrode assemblies 19, 20. As best seen in FIG.-3, each electrode assembly comprises a tungsten wire or shank 21 around which are disposed .a pair of generally concentric coils or helices of tungsten wire. The inner coil 22 is tightly wound around the shank and the outer coil 23 is screwed over the inner coil. The ends of the inner coil and the entire outer coil are close wound. The inner coil may have a greater winding :pitch in its central portion in order to provide cavities of greater volume than the. ordinary interstices between the wires; the cavities or interstices are subsequently filled with the electroncmitting mixture or activating materialas indicated .at 24.

Prior to sealing the envelope to the stem assembly, the

electrodes are dipped into a suspension of material which can be heat-treated to yield alkaline earth oxides or metals including barium. For instance, the electrodes may be dipped into a suspension ofbarium thorate with added thorium, or barium zirconate with added zirconium, as

disclosed in'copending application Serial. No. 860,130, filed December 17, 1959, now US. Patent No. 3,188,236,

of Dimitrios M. Speros, entitled Cathodes and Method of Manufacture, and assigned to the same assignee as the present invention. The suspension-may.consist'of the material in a volatile organic liquid such as bntyl acetate or ethyl alcohol. The. material coats the assem: bly and also fills the cavities and interstices between the turns of the coils. The cathodes are then activated by heating them to a temperature of about 800 C. or more while the device is being pumped under an exhaust'systern to remove the gases and vapors which are released during the processing. The device is completed by thoroughly evacuating and outgassing, providing a filling of an inert gas, and then tipping off the exhaust tube as shown at 25.

The breakdown voltage of the spark tube depends upon the nature and pressure .of the filling gas and is also inand most convenient to use.

within tolerances which can be maintained during manufacture. lowers the breakdown voltage.

or spark discharge. At fill pressures higher than 500 millimeters of mercury, the electrode ,spacing required for the desired range of breakdown voltage: becomes too small for reproducibilitypand practical manufacture. The spacing between the electrode assemblies, measured between the surfaces of the outside, coils 23, is in the range of 0.1 to 5 millimeters.

As an example of a spark tube embodying the invention, I use for the envelope 13 a glass envelope measuring about inch in diameter by /1 inch in length of the kind commonly used for the glow switches of fluorescent lamp 1 The shank of the electrode is 22 mil diameter 1 1% thoriated tungsten wire; the inner coil is 14 mil starters.

diameter tungsten wire; and the .outer' coil is 17 mil diameter tungsten wire. The are gap or spacing between the electrode assemblies is 0.4 millimeter and the filling consists :or argon gas at a pressure of millimeters of mercury. This results in a spark tube having a breakdown voltage of 130 to 140 volts and a maintaining voltage of about 10 to 15 volts. Under these conditions the loss of energy in the spark tube is quite small, being about 10% to, 15% while to 85% of the energy stored in the pulsing condsensers is effectively transferred Under life .test, the spark.

into the pulsetransformer. 7 tube continued to operate successfully after cycling to an extent corresponding to the number of starts anticipated over the normal life of a high pressure sodium alumina lamp, such being in excess of several thousand hours.

In general, I have found that electrode spacings in the range of 0.25 to. 1.0 millimeter are most satisfactory; for

the range of breakdown voltages of to 300 volts desired in connection with alkali vapor alumina lamps, combined with argon -fill pressures in the' range of 50 to 300 millimeters of mercury.

While a specific embodiment of the invention has'been illustrated and described in detail, it is intended as illustrative and not in order to limit the invention'fthereto.

The scope of the invention is to be determined ,by the appended claims which are intended to cover any'modifications falling within its spirit.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A spark tube comprising a vitreous enveloperhaving a pair of lead-in wires sealedthereintothrough-fa re-entrant stem and containing a filling of an inert gas' 5 at a pressure inthe range of 2 5 to 500 millimeters of, 'mercury, a pair of electrodeassemblies. attached to said :7

lead-ins and extending side 'by. side within said envelope at an interelectrode distance apart not exceeding 5 milliin the coiling.

2. A spark tube having a low maintaining voltage comprising an envelope containing a filling ;of an inert gas at a pressure in the range of 25 to 500 millimete'rsof a mercury and having sealed therein a pairof electrode assemblies, each electrode assembly comprising a tungsten shank having a pair of tungsten wire coils wound there over one over the other, and activating material jcoat ing said electrode assemblies and filling the interstices Decreasing the fill pressure of. the inert gas. However a. fill pressure of 25 millimeters of mercury isa lower limit because. lower fill pressures result in a glow discharge with high voltage drop rather than the desired low voltage are I between the turns of said coils, said activating material comprising alkaline earth interoxides including barium oxide and barium, said electrode assemblies extending parallel and side by side with a separation between their most proximate points in the range of 0.1 to 5 millimeters.

3. A spark tube having a breakdown voltage in the range of 100 to 300 volts and a low maintaining voltage in the range of to volts comprising an envelope containing a filling of argon at -a pressure in the range of 50 to 300 millimeters of mercury and having sealed therein a pair of electrode assemblies, each electrode assembly comprising a tungsten shank having tungsten wire coiled thereon, and activating material coating said each electrode assembly and filing the interstices between turns of the coiling, said activating material comprising alkaline earth interoxides including barium oxide and barium, said electrode assemblies extending parallel and side by side with a separation between their most proximate points in the range of 0.25 to 1.0 millimeter.

4. A spark tube having a breakdown voltage in the range of 100 to 300 volts and a low maintaining voltage in the range of 10 to 15 volts comprising an envelope containing :a filling of argon at a pressure in the range of 50 to 300 millimeters of mercury and having sealed therein a pair of electrode assemblies, each electrode assembly comprising a tungsten shank having a pair of tungsten wire coils wound thereon one over the other, the inner coil being open wound in its central portion while its ends and the outer coil are substantially close wound whereby to leave interstices of substantial volume between the turns of the inner coil, and activating material coating said each electrode assembly and filling said interstices, said activating material comprising alkaline earth interoxides including barium oxide and :barium, said electrode assemblies extending parallel and side by side with a separation between their most proximate points in the range of 0.25 to 1.0 millimeter.

*5. A spark tube as defined in claim 1 wherein the refractory metal is tungsten.

References Cited by the Examiner UNITED STATES PATENTS 2,098,113 11/1937 Spaeth 313183 2,122,932 7/ 1938 Dufiendack et a1. 3l3214 X 2,886,737 5/1959 Fruengel 313-217 X 3,132,409 5/1964 Freeman 313-346 X OTHER REFERENCES Cobine: Gaseous Conductors, McGr-aW-Hill Book Co., New York, 1941.

JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, Examiner. K. CROSSON, R. L. JUDD, Assistant Examiners. 

1. A SPARK TUBE COMPRISING A VITREOUS ENVELOPE HAVING A PAIR OF LEAD-IN WIRES SEALED THEREINTO THROUGH A RE-ENTRANT STEM AND CONTAINING A FILLING OF AN INERT GAS AT A PRESSURE IN THE RANGE OF 25 TO 500 MILLIMETERS OF MERCURY, A PAIR OF ELECTRODE ASSEMBLIES ATTACHED TO SAID LEAD-INS AND EXTENDING SIDE BY SIDE WITHIN SAID ENVELOPE AT AN INTERELECTRODE DISTANCE APART NOT EXCEEDING 5 MILLIMETERS, EACH ELECTRODE ASSEMBLY COMPRISING A REFRACTORY METAL SHANK WITH REFRACTORY METAL WIRE COILED THEREOVER AND BEING ACTIVATED WITH ELECTRON-EMITTING MATERIAL COMPRISING ALKALINE EARTH OXIDE AND METAL, SAID MATERIAL COATING THE ELECTRODE ASSEMBLY AND FILLING THE INTERSTICES IN THE COILING. 